1. Field of the Invention
The present invention relates to a vehicle lighting device constructed such that light emitted from a light emitting element such as a light emitting diode exits forward relative to the lighting device by means of a light transmitting member arranged on the front side of the lighting device.
2. Background Art
In the prior art, for example, as described in “Patent Document 1”, a vehicle lighting device is known in which light emitted from a light emitting element that is arranged near a predetermined point on the optical axis extending in the forward and backward directions of the lighting device and that is directed in the forward direction of the lighting device exits forward relative to the lighting device by means of a light transmitting member arranged on the front side of the lighting device.
This vehicle lighting device is constructed such that light emitted from the light emitting element enters the light transmitting member, then undergoes internal reflection in the front surface, then undergoes internal reflection again in the rear surface, and then exits the front surface. At that time, a center region in the front surface of the light transmitting member is mirror-finished for the purpose of internal reflection of the light emitted from the light emitting element.
Further, “Patent Document 2” describes an optical apparatus constructed such that light emitted from the light emitting element enters the light transmitting member, then undergoes internal reflection in the front surface, then undergoes internal reflection again in the rear surface, and then exits the front surface, and that a center region in the front surface of the light transmitting member is formed in the shape of a convex lens so as to deflect the outgoing light having exited the light emitting element and reached the center region.    [Patent Document 1] JP-A-2005-11704    [Patent Document 2] JP-A-2002-94129
By adopting the configuration described above in “Patent Document 1”, a vehicle lighting device can be constructed with a reduced thickness.
Nevertheless, in the vehicle lighting device described in “Patent Document 1”, mirror finish is performed in the center region in the front surface of the light transmitting member. This causes a problem that a part of the light having exited the light emitting element and undergone internal reflection in the center region is not utilized as forward illuminating light and hence the utilization factor of the source light flux cannot satisfactorily be improved.
In contrast, as in the optical apparatus described above in “Patent Document 2”, when a center region in the front surface of the light transmitting member is formed in the shape of a convex lens, almost the entirety of the light having exited the light emitting element and reached the front surface of the light transmitting member can be utilized as forward illuminating light. Thus, the utilization factor of the source light flux can satisfactorily be improved.
Nevertheless, in the optical apparatus described in “Patent Document 2”, the position of the outer peripheral edge of the center region is set to be near a position where the incident angle of the light having exited the light emitting element and reached the front surface of the light transmitting member is equal to a critical angle. Thus, the fraction of the light that exits the center region increases, while the fraction of the light that undergoes internal reflection in the front surface of the light transmitting member and then undergoes internal reflection in the rear surface decreases. Thus, when this optical apparatus is used as a lighting device, the following problem arises.
That is, a light source image formed by the light that has undergone internal reflection in the front surface of the light transmitting member, then undergone internal reflection again in the rear surface, and then exited the front surface is small. In contrast, a light source image formed by the light having directly exited the center region in the front surface of the light transmitting member is large. Thus, a problem arises that when the fraction of the light that exits the center region is relatively excessive, the light distribution pattern formed on a virtual vertical screen located in front of the lighting device cannot be formed in the shape of a light distribution pattern having a high center luminosity.